Abstract
The high-value utilization of valuable components in spent lithium iron phosphate (LFP) batteries is of dual significance for resource recycling and environmental protection. This study systematically investigates the effects of sulfuric acid concentration, reaction temperature, particle size, and stirring speed on the leaching behavior of lithium (Li) and iron (Fe) from LFP electrode powder. The optimal conditions for maximum Li and Fe extraction rates are determined, and the leaching kinetics are analyzed to elucidate the underlying mechanism. The results indicate that under optimized conditions (sulfuric acid concentration of 2.5 mol/L, leaching temperature of 75°C, particle size of approximately 0.14 mm, and stirring speed of 400 rpm), both Li and Fe exhibit leaching efficiencies exceeding 99.8%. The leaching process is found to be controlled by external diffusion, with the leaching kinetics following the Avrami model. The apparent activation energies for Fe and Li during the leaching process are determined to be 11.03 kJ/mol and 8.45 kJ/mol, respectively. This study provides a theoretical basis for the simultaneous extraction of valuable elements from spent LFP electrode powders.
Keywords
Introduction
Lithium iron phosphate (LFP) batteries have gained significant popularity in the electric vehicle and energy storage sectors due to their excellent cycling performance, high safety, and low production costs. However, the increasing use of LFP batteries has led to a considerable accumulation of spent batteries, posing challenges related to waste management and resource recovery. As the typical lifespan of LFP batteries ranges from 5 to 8 years, their waste generation is expected to surge in the coming years. By 2030, the weight of spent LFP batteries is projected to reach 313,000 tonnes [1]. Improper disposal of these batteries can not only lead to the waste of valuable resources such as Li, Fe, and P but also pose potential environmental hazards due to the presence of toxic fluorine-containing components in their electrolytes [2-4].
Hence, developing safe disposal and resource recovery technologies for spent LFP batteries is crucial for environmental protection and mitigating global climate change and resource supply issues [5-6]. Currently, two primary routes exist for the regeneration of spent LFP electrodes: direct repair and hydrometallurgical processing. Direct repair involves repairing the defects in spent LFP active materials without disrupting their original crystal lattice structure, restoring their electrochemical performance. Although this method is relatively simple and cost-effective, the consistency of the repaired LFP positive electrode materials often falls short of that required for new power electrodes [7-10].
In contrast, hydrometallurgical processing has emerged as the preferred route for recycling LFP electrode materials due to its flexibility, versatility, and high resource recovery efficiency. This process typically involves four steps: pretreatment, leaching of valuable metals, purification of the leachate, and product preparation (metal salts or cathode materials) [11-13]. Sulfuric acid is widely used as a leaching agent due to its low cost, minimal environmental impact, and effective leaching ability [14-17].
This study focuses on the systematic investigation of the leaching behavior and kinetics of Li and Fe from spent LFP electrode powders using sulfuric acid as the leaching agent. The effects of various parameters, including sulfuric acid concentration, reaction temperature, particle size, and stirring speed, are evaluated to optimize the leaching conditions. Furthermore, the leaching kinetics are analyzed to elucidate the underlying mechanisms and determine the apparent activation energies for Li and Fe leaching.
Materials and Methods
Materials
Spent LFP electrode materials were obtained from a storage technology company. The electrodes were first discharged using a 100 g/L NaCl solution for 54 hours to reduce the residual voltage below 0.8 V. Subsequently, the electrodes were manually disassembled to separate the aluminum casing, positive electrode sheets, separators, and negative electrode sheets. The positive and negative electrode sheets were cut into 4 cm × 6 cm pieces and oxidized at 500°C for 0.5 hours in an air atmosphere using a tube furnace. After oxidation, the electrode sheets were separated into current collectors and electrode powders through vibration sieving. The main elemental composition of the obtained spent LFP electrode powder was determined to be P (15.9%), Fe (38.5%), Li (3.76%), C (11.2%), and minor amounts of Al (0.38%), Cu (1.92%), and F (0.27%). The X-ray diffraction (XRD) pattern of the powder is presented in Figure 1, revealing its primary phases as C, LiFePO4, Li3Fe2(PO4)3, and Fe2O3.
Experimental Procedure
The leaching experiments were conducted in a 1000 mL three-necked flask equipped with a mechanical stirrer and a temperature-controlled water bath. A predetermined volume of sulfuric acid solution (800 mL) with a specified concentration (1.5–3.5 mol/L) was added to the flask and heated to the desired reaction temperature (55–95°C). Once the target temperature was reached, 8 g of spent LFP electrode powder (sieved to a specified particle size range of 0.095–0.375 mm) was added to the flask at a liquid-to-solid ratio of 100 mL/g. The reaction mixture was stirred at a predetermined speed (200–600 rpm), and samples were collected at regular intervals (0.5, 2, 4, 8, 10, 15, 30, 45, 60, 90, and 120 minutes) for analysis. After solid-liquid separation, the concentrations of Fe and Li in the leachate were determined, and the leaching efficiencies were calculated using Equation 1:
textLeachingEfficiency(%)=(m0×WiCi×V)×100
where Ci is the concentration of Fe or Li in the leachate (g/L), V is the volume of the leachate (L), m0 is the mass of the spent LFP electrode powder (g), and Wi is the mass fraction of Fe, Li, Cu, or Al in the electrode powder (%).
Characterization Methods
The concentrations of Fe and Li in the leachate were analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima 5300DV, PerkinElmer, USA). The carbon content was determined using an infrared carbon and sulfur analyzer, while the fluorine content was measured using a fluoride ion-selective electrode. The phase composition of the spent LFP electrode powder was characterized using X-ray diffraction (XRD, Ultima IV, Rigaku, Japan).
Results and Discussion
Effect of Sulfuric Acid Concentration
The influence of sulfuric acid concentration on the leaching behavior of Fe and Li from spent LFP electrode powder was evaluated at a reaction temperature of 75°C, a liquid-to-solid ratio of 100 mL/g, an average particle size of 0.14 mm, a stirring speed of 400 rpm, and a leaching time of 120 minutes. The results are presented.
As shown in Figure 2(a), the leaching efficiency of Fe gradually increased with increasing sulfuric acid concentration, reaching a peak leaching efficiency of approximately 100% after 90 minutes of reaction at sulfuric acid concentrations of 2.5 mol/L and above. In contrast, the leaching of Li was more rapid, with over 80% of Li being leached within the first 0.5 minutes of reaction (Figure 2(b)). Complete leaching of Li was achieved within 10 minutes, indicating that Li is more readily leached than Fe. Based on these results, a sulfuric acid concentration of 2.5 mol/L was selected as optimal for simultaneous leaching of Fe and Li.
Effect of Reaction Temperature
The effect of reaction temperature on the leaching behavior of Fe and Li was investigated at a sulfuric acid concentration of 2.5 mol/L, a liquid-to-solid ratio of 100 mL/g, an average particle size of 0.14 mm, a stirring speed of 400 rpm, and a leaching time of 120 minutes.
The reaction temperature significantly impacted the leaching behavior of both Fe and Li. As the temperature increased, the leaching efficiencies of both metals improved. At 55°C, Li was almost completely leached within 16 minutes, while Fe achieved a leaching efficiency of approximately 80% under the same conditions. Complete leaching of Fe was achieved at temperatures of 75°C and above, with leaching efficiencies approaching 100% after 90 minutes of reaction. However, maintaining higher temperatures necessitates greater energy consumption, and the evaporation of the leachate increases significantly above 80°C, posing safety concerns. Therefore, 75°C was selected as the optimal leaching temperature.
Effect of Particle Size
The influence of particle size on the leaching behavior of Fe and Li was examined at a sulfuric acid concentration of 2.5 mol/L, a reaction temperature of 75°C, a liquid-to-solid ratio of 100 mL/g, a stirring speed of 400 rpm, and a leaching time of 120 minutes. The results are shown in Figure 4.
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Figure 4: Effect of particle size on the leaching efficiency of (a) Fe and (b) Li.
The particle size played a crucial role in the leaching process. Smaller particle sizes led to higher specific surface areas, promoting faster reaction rates between the sulfuric acid and the battery material. Additionally, smaller particles facilitated the penetration of the leaching solution into the interior of the LFP material, shortening the diffusion path for dissolved products. Consequently, as the particle size decreased from 0.375 mm to 0.095 mm, the leaching efficiencies of both Fe and Li gradually increased. Nearly complete leaching of Li (above 99.9%) was achieved within 8 minutes, while Fe leaching approached equilibrium after 90 minutes, with efficiencies exceeding 97.1%. Considering the energy consumption associated with milling smaller particles, a particle size of approximately 0.14 mm was deemed optimal, as most electrode active materials fell within this range and could be directly leached without further grinding.
Effect of Stirring Speed
The impact of stirring speed on the leaching behavior of Fe and Li was investigated at a sulfuric acid concentration of 2.5 mol/L, a reaction temperature of 75°C, a liquid-to-solid ratio of 100 mL/g, an average particle size of 0.14 mm, and a leaching time of 120 minutes. The results are presented in Figure 5.
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Figure 5: Effect of stirring speed on the leaching efficiency of (a) Fe and (b) Li.
Increasing the stirring speed enhanced mass transfer and intensified the leaching process. As the stirring speed was raised from 200 rpm to 600 rpm, the leaching efficiencies of both Fe and Li improved, indicating that the leaching kinetics were diffusion-controlled. Complete leaching of Li (approximately 99.9%) was achieved within 8 minutes, while Fe leaching approached equilibrium (approximately 97%) after 90 minutes. Based on leaching efficiency and economic considerations, a stirring speed of 400 rpm was selected as optimal.
Leaching Kinetics Study
The leaching process can be viewed as the reverse of a crystallization process. Assuming random nucleation and a homogeneous unreacted mass, the Avrami equation (Equation 2) has been successfully applied to describe the leaching kinetics of multiple metals in solid-liquid reactions [18-20].
−ln(1−x)=ktn
where x is the volume fraction of the leached material, k is the leaching rate constant, t is the leaching time (minutes), and n is the reaction characteristic parameter.
Plotting ln[−ln(1−x)] versus lnt yields a linear relationship, with the slope equal to n and the intercept corresponding to lnk. As shown in Figure 6, a good linear correlation was observed under various conditions, confirming the applicability of the Avrami equation in describing the leaching kinetics of spent LFP electrode powder in sulfuric acid.
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Figure 6: Linear relationship between ln[−ln(1−x)] and lnt under different conditions.
Table 1 summarizes the leaching kinetic parameters calculated using the Avrami equation under varying rotation speeds, acid concentrations, temperatures, and particle sizes. The average values of the reaction characteristic parameter n for Fe and Li were 0.2349 and 0.2867, respectively, both less than 0.5, indicating that the leaching processes of Fe and Li are controlled by external diffusion.
| Condition | Fe | Li |
|---|---|---|
| 1.5 mol/L H2SO4 | n = 0.1502 | n = 0.2398 |
| ln k = -0.4823 | ln k = 0.7185 | |
| 2.0 mol/L H2SO4 | n = 0.1812 | n = 0.2460 |
| ln k = -0.3626 | ln k = 0.7971 | |
| 2.5 mol/L H2SO4 | n = 0.2310 | n = 0.2852 |
| ln k = -0.2540 | ln k = 0.8548 | |
| 3.0 mol/L H2SO4 | n = 0.2513 | n = 0.3264 |
| ln k = -0.1550 | ln k = 0.9399 | |
| 3.5 mol/L H2SO4 | n = 0.2728 | n = 0.3538 |
| ln k = -0.0287 | ln k = 1.0075 | |
| 55°C | n = 0.1325 | n = 0.1025 |
| ln k = -0.4694 | ln k = 0.6013 | |
| 65°C | n = 0.1857 | n = 0.2103 |
| ln k = -0.3370 | ln k = 0.7415 | |
| 75°C | n = 0.2352 | n = 0.2896 |
| ln k = -0.2540 | ln k = 0.8548 | |
| 85°C | n = 0.2765 | n = 0.3246 |
| ln k = -0.1642 | ln k = 0.9108 | |
| 95°C | n = 0.3875 | n = 0.3632 |
| ln k = -0.0343 | ln k = 0.9781 | |
| 0.375 mm | n = 0.1882 | n = 0.1650 |
| ln k = -0.4496 | ln k = 0.7278 | |
| 0.227 mm | n = 0.1812 | n = 0.2303 |
| ln k = -0.3376 | ln k = 0.8116 | |
| 0.140 mm | n = 0.2310 | n = 0.2852 |
| ln k = -0.2540 | ln k = 0.8548 | |
| 0.113 mm | n = 0.2711 | n = 0.3168 |